This invention relates to methods and devices used to measure the liquid levels in a cryogenic environment. More specifically, the invention relates to a new and improved method and apparatus for assembling a liquid helium level sensor that is used for measuring the liquid helium level in a cryogenic cooler.
Commonly used liquid helium sensors employ a string of superconducting NbTi filament in either rigid or flexible tubing. Generally, filament sizes range from 0.0005 inch to 0.002 inch in diameter. The sensor operates by measuring the resistance of the portion of the superconductor filament that is above the liquid helium level. The portion that is submerged in the liquid will not contribute measurable resistance because it is in the superconducting state at the temperature of liquid helium. Very thin NbTi filament is employed so that the resistance generated by that portion of the filament above the liquid helium surface produces a measurable voltage drop across the entire filament even with a current as small as 50 mA to 100 mA. The liquid helium level is then inversely proportional to the voltage drop measured, given a constant excitation current.
In general, the sensitivity and accuracy of the sensor increases as the resistance of the superconducting filament increases. Normally, there are two ways to increase the resistivity of the filament. The first way is to reduce the diameter of the filament. The second method involves etching off the copper matrix from the NbTi—Cu filament, because copper has very low resistivity.
In practice, very thin filament is difficult to handle, easy to break and is so thin that is extremely difficult to manufacture. Also, very thin filament size reduces the current carrying capacity of the superconducting filament, thus reducing the voltage output. This causes a reduction of the signal to noise ratio and reduces the accuracy of the measuring device.